In recent years, disc-type brakes for vehicles have increasingly supplanted the previously conventional drum-type vehicle brakes. It has been proposed that disc-type brakes could be improved by constantly coating the friction material on the discs with oil. Such a brake is referred to as a "wet disc brake".
There are several advantages which may be achieved with a wet-type disc brake as opposed to a dry-type disc brake. One advantage is that by coating the friction material with oil, the coefficient of friction between the discs is more stable with a rise in temperature than is the case where the friction material is dry. As dry friction material becomes heated, the coefficient of friction between it and an engaged metal surface drops, resulting in decreasing braking effect. This phenomena is frequently referred to as "fade". Fading of braking ability may occur upon continued actuation of a brake.
Another advantage to be gained by a wet disc brake is that the friction material will not wear out as fast as is the case where the friction material is dry. This results in a longer lasting brake. A further advantage of such brakes is that they are, of necessity, enclosed in a sealed housing. The friction material generally includes asbestos fiber. As these fibers break away during wear of the friction material, they may float in the air and escape to the ambient atmosphere as is the case with the standard dry friction material brakes which are contained in an unsealed housing. In the wet disc brake environment, the asbestos fibers are absorbed into the oil contained in the casing and do not escape to the ambient atmosphere to cause an air pollution problem.
Wet disc brakes also result in the advantages of economy of space for a given torque capacity, provide a generous lining area for friction material, and provide consistent performance in practically any operating environment.
The wet disc brake of the present invention includes several features not previously believed to have been proposed in connection with said such structures. Firstly, the discs on which the friction material is mounted are circulated in a pool of oil provided in the lower portion of the casing in which the discs are contained. These discs, which rotate with the axle, have the lower portions thereof passed through the oil bath and oil is carried by grooves and slots in the friction surfaces so that such surfaces are continually wetted. Previously known systems have proposed that the oil be applied to the friction material by means of a spray or other injection device generally provided in the upper portion of the brake casing.
Another feature of the present brake is the provision of a spring means biasing the piston, which is provided to force the brake discs together for braking action, against the disc stack at all times preventing the piston from retracting from the discs during non-braking periods. In normal operation, there is a tendency for the piston to retract due to rotation of the brake discs. This spring pressure results in both the discs and piston being in position for immediate actuation of the brake when desired. The piston and discs do not have to move any significant distance to achieve braking action. This results in the piston and discs being self-adjusting to allow for wear and for initial manufacturing tolerances of the brake pack. This does result in a slight drag on the brakes but the energy loss and wear are insignificant.
Further, in one embodiment of the invention, the disc brake is intergrated and made an extension of a sealed housing containing oil which is provided as part of the power drive of the vehicle for which the brake is provided. Oil flows from the vehicle housing into the brake housing for the desired purposes previously enumerated. A baffle is provided between the two housings to constantly maintain a pool of oil in the brake housing regardless of tilting of the vehicle housing which may occur as the vehicle itself tilts. Such tilting could drain off all the oil from the brake housing if the baffle were not provided.